Polyhalite is a mineral having the formula K2Ca2Mg(SO4)4.2H2O, which occurs naturally in, for example, deposits in Texas, New Mexico, Ukraine, and Germany. Ore deposits are conventionally sub-surface mined to produce ore in rock or chunk form because polyhalite is not sufficiently water-soluble to allow ore deposits to be economically solution-mined.
Polyhalite may be used in the production of various salts, such as potassium sulfate (also known as sulfate of potash or SOP), potassium magnesium sulfate, potassium calcium sulfate, potassium hydroxide, magnesium sulfate, etc. Some potassium salts are important ingredients in fertilizers and feedstocks for various industrial processes.
Polyhalite is soluble or leachable in aqueous solutions without calcining (i.e., heating to a temperature at which the polyhalite at least partially dissociates), but dissolution is relatively slow. Several methods of processing polyhalite ores are known, such as those described in John E. Conley and Everett P. Partridge, “Potash Salt from Texas-New Mexico Polyhalite Deposits: Commercial Possibilities, Proposed Technology, and Pertinent Salt-Solution Equilibria,” U.S. Dept. of the Interior Bureau of Mines Bulletin 459 (1944), which is incorporated herein in its entirety by this reference. For example, polyhalite may be calcined by methods known in the art, and K2SO4 and MgSO4 may be extracted by hot water, cold water, or other methods. The resulting liquor may be subjected to various processes in order to yield products such as potassium sulfate (SOP or K2SO4), leonite (K2SO4MgSO4.4H2O), schoenite (K2SO4MgSO4.6H2O), langbeinite (K2SO4.2MgSO4), kieserite (MgSO4.H2O), epsomite (MgSO4.7H2O), etc.
Known methods of processing polyhalite ores generally require significant power and/or steam inputs, and may have process limitations. For example, in some processes, K2SO4 may be recovered with an efficiency of about 86%, but may not be in the form conventionally used as fertilizer (e.g., crystalline form, purity, etc.). In some processes, potassium may be produced entirely as SOP, but the efficiency may be only about 74%, and steam and power requirements may be relatively higher. In still other processes, K2SO4 efficiency may be as high as 95%, but with higher steam and power requirements. Some processes may yield sulfate-containing products in less-than-ideal ratios (i.e., in ratios that do not maximize economic value of products). It would therefore be advantageous to provide a method of processing polyhalite that minimizes or alleviates these shortcomings.